The interest in deploying low-power nodes, such as pico base stations, home eNodeBs, relays, remote radio heads, etc., for enhancing the macro network performance in terms of the network coverage, capacity and service experience of individual users has been constantly increasing over the last few years. At the same time, there has been realized a need for enhanced interference management techniques to address the arising interference issues caused, for example, by a significant transmit power variation among different cells and cell association techniques developed earlier for more uniform networks
In 3rd Generation Partnership Project, 3GPP, heterogeneous network deployments have been defined as deployments where low-power nodes of different transmit powers are placed throughout a macro-cell layout, implying also non-uniform traffic distribution. Such deployments are, for example, effective for capacity extension in certain areas, so-called traffic hotspots, i.e. small geographical areas with a higher user density and/or higher traffic intensity where installation of pico nodes can be considered to enhance performance. Heterogeneous deployments may also be viewed as a way of densifying networks to adopt for the traffic needs and the environment. However, heterogeneous deployments bring also challenges for which the network has to be prepared to ensure efficient network operation and superior user experience. Some challenges are related to the increased interference in the attempt to increase small cells associated with low-power nodes, also known as cell range expansion; the other challenges are related to potentially high interference in uplink due to a mix of large and small cells.
According to 3GPP, heterogeneous deployments consist of deployments where low power nodes are placed throughout a macro-cell layout. User equipments, UEs, being served by a low power radio base station are generally said to belong to a Closed Subscriber Group, CSG, for that particular low power radio base station. The interference characteristics in a heterogeneous deployment can be significantly different than in a homogeneous deployment, in downlink or uplink or both. Examples hereof are given in FIG. 1.
FIG. 1 illustrates a macro radio base station 100 having a coverage area 101, generally known as a cell 101. A cell of a macro radio base station is also referred to as a macro cell. Within the cell 101 of the macro radio base station, three low power radio base stations 110, 120 and 130 are deployed. The low power radio base stations have a respective associated cell 111, 121 and 131, also referred to a low power cells. FIG. 1 further illustrates one UE 115, 125 and 135 being present in each of the low power cells 111, 121 and 131. The UEs 115 and 125 in FIG. 1 are both being served by the macro radio base station 100 even though the UEs are located within the cells 111 and 121, and the UEs are referred to as macro UEs. This means that the UEs 115 and 125 have no access to the respective CSGs of the respective low power radio base stations 110 and 120. UE 135 belongs to a CSG of low power radio base station 130 and is hence not being served by the macro radio base station 100, and the UE 135 is referred to as a CSG UE. In FIG. 1, in case (a) the macro UE 115 will be interfered by the low power radio base station 110 when being served by the macro radio base station 100. In case (b) the UE 125 causes severe interference towards the low power radio base station 120, and in case (c), the CSG UE 135 is interfered by the low power radio base station 120. In some examples, a low power radio base station may also be referred to as a HeNB, short for Home eNode B. Other examples of low power nodes are pico base station, micro base station, and medium-range base station. The low-power nodes may or may not operate in the CSG mode.
Another challenging interference scenario occurs with so-called cell range expansion, when the traditional downlink cell assignment rule diverges from the Reference Signal Received Power, RSRP, based approach, e.g. towards pathloss or path gain based approach, e.g., when adopted for radio base stations with a transmit power lower than that of a neighbour radio base station. The idea of the cell range expansion is illustrated in FIG. 2 where the cell range expansion of a low power cell is implemented by means of a delta-parameter and the UE 115, 125, 135 potentially can “see” a larger low power cell coverage area when a positive delta-parameter is used in cell selection/reselection. The cell range expansion is limited by the downlink, DL, performance since uplink, UL, performance typically improves when the cell sizes of neighbour cells become more balanced.
To ensure reliable and high bit rate transmissions as well as robust control channel performance, maintaining a good signal quality is a must in wireless networks. The signal quality is determined by the received signal strength and its relation to the total interference and noise received by the receiver. A good network plan, which, among the others also includes cell planning, is a prerequisite for the successful network operation, but it is static. For more efficient radio resource utilization, it has to be complemented at least by semi-static and dynamic radio resource management mechanisms, which are also intended to facilitate interference management, and deploying more advanced antenna technologies and algorithms.
One way to handle interference is, for example, to adopt more advanced transceiver technologies, e.g. by implementing interference cancellation mechanisms in terminals. Another way, which can be complementary to the former, is to design efficient interference coordination algorithms and transmission schemes in the network. The coordination may be realized in static, semi-static or dynamic fashion. Static or semi-static schemes may rely on reserving time-frequency resources (e.g., a part of the bandwidth and/or time instances) that are orthogonal for strongly interfering transmissions. Dynamic coordination may be implemented e.g. by means of scheduling. Such interference coordination may be implemented for all or specific channels (e.g., data channels or control channels) or signals.
For heterogeneous deployments, there have been standardized enhanced inter-cell interference coordination (eICIC) mechanisms for ensuring that the UE subject to high interference is able to perform at least some measurements (e.g. Radio Resource Management, RRM, Radio Link Monitoring, RLM, and Channel State Information, CSI, measurements) in special low-interference subframes. These mechanisms involve configuring patterns of reduced power and/or reduced activity subframes (also referred to Almost Blank Subframes, ABS) at transmitting nodes and configuring measurement patterns for UEs.
Two types of patterns have been defined for eICIC to enable restricted measurements in DL: restricted measurement patterns, which are configured by a network node and signalled to the UE; and transmission patterns (also known as ABS patterns), which are configured by a network node, describe the transmission activity of a radio node, e.g. a radio base station, and may be exchanged between the radio nodes
In general, in Long Term Evolution, LTE, the UL interference is coordinated by means of scheduling and UL power control, where the UE transmit power is configured to meet a certain Signal to Noise Ratio, SNR, target which may be further fine tuned by a few other related parameters. Both scheduling and UL power control allow for coordinating the UL interference in time, frequency and space.
It is mandatory for all UEs to support all intra-Radio Access Technology, RAT, measurements (i.e. inter-frequency and intra-band measurements) and meet associated requirements. However the inter-band and inter-RAT measurements are UE capabilities, which are reported to the network during the call setup. The UE supporting certain inter-RAT measurements should meet the corresponding requirements. For example a UE supporting LTE and Wideband Code Division Multiple Access, WCDMA, should support intra-LTE measurements, intra-WCDMA measurements and inter-RAT measurements (i.e. measuring WCDMA when serving cell is LTE and measuring LTE when serving cell is WCDMA). Hence a communication network can use these capabilities according to its strategy. These capabilities are highly driven by factors such as market demand, cost, typical network deployment scenarios, frequency allocation, etc.
The UE may be configured to perform positioning measurements. For example, for UE-assisted Observed Time Difference of Arrival, OTDOA, positioning, the UE receives assistance data from a positioning node (e.g. Evolved Serving Mobile Location Centre, E-SMLC, in LTE), where the assistance data comprises a list of cells, including a reference cell, for which the UE will perform Reference Signal Time Difference, RSTD, measurements and report the measurements to the positioning node.
To enable positioning in LTE and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, physical signals dedicated for positioning (Positioning Reference Signals, or PRSs [3GPP TS 36.211]) have been introduced and low-interference positioning subframes have been specified by 3GPP.
PRS are transmitted from one antenna port (R6) according to a pre-defined pattern. A frequency shift, which is a function of Physical Cell Identity, PCI, can be applied to the specified PRS patterns to generate orthogonal patterns and modelling the effective frequency reuse of six, which makes it possible to significantly reduce neighbour cell interference on the measured PRS and thus improve positioning measurements. Even though PRS have been specifically designed for positioning measurements and in general are characterized by better signal quality than other reference signals, the standard does not mandate using PRS. Other reference signals, e.g. cell-specific reference signals (CRS) may also be used for positioning measurements, although no requirements are defined for CRS-based RSTD measurements.
PRS are transmitted in pre-defined positioning subframes grouped by several consecutive subframes (NPRS), i.e. one positioning occasion. Positioning occasions occur periodically with a certain periodicity of TPRS subframes, i.e. the time interval between two positioning occasions. The standardized periods TPRS are 160, 320, 640, and 1280 ms, and the number of consecutive subframes may be 1, 2, 4, or 6. The positioning occasion configuration parameters the UE receives in the OTDOA assistance data signalled by the network. It is up to the network to ensure low-interference conditions in the positioning subframes configured for UE RSTD measurements.
For each cell in the assistance data, the UE will perform RSTD measurements in the indicated positioning subframes that contain PRS. For CRS-based RSTD measurements in heterogeneous network, the UE may perform measurements in the restricted positioning subframes if the corresponding patterns are known to the UE.
When the UE is configured with inter-frequency RSTD measurements, the positioning measurement occasions may be further restricted by measurement gap patterns. For inter-frequency RSTD measurements, measurements gap pattern #0, where measurement gaps are repeated every 40 ms period, has to be configured by the network.
In LTE, the following Enhanced Cell Id, E-CID, positioning measurements may be performed by the UE: RSRP measurements for serving and neighbour cells; Reference Signal Received Quality, RSRQ, measurements for serving and neighbour cells; and UE Reception-Transmission, Rx-Tx, timing difference measurements for the serving cell or serving radio base station. Given that the above measurements are performed on CRS, in heterogeneous deployments the UE is likely to perform these measurements also in the restricted measurement subframes configured by eICIC.
The radio base station, or eNodeB in LTE, may also perform E-CID measurements, e.g. eNodeB Rx-TX (Timing Advance Type 1), Timing advance Type 2 and Angle of Arrival, AoA, measurements. Note that Timing Advance measurements are also used for configuring UE timing adjustment for general operation, i.e., not related to positioning.
Some positioning measurements such as UE Rx-Tx time difference, eNodeB Rx-Tx time difference, Timing Advance, TA, AoA, Uplink Time Difference of Arrival, UTDOA measurements, etc. require measurements on the uplink transmitted signals (e.g. Sounding Reference Signals, SRS, demodulation reference signals, UE-specific reference signals or channels (e.g. Random Access Channel, RACH).
In Evolved Universal Mobile Telephone System Terrestrial Radio Access, E-UTRAN, the serving cell or the serving radio base station can request the UE to acquire the cell global identifier, CGI, which uniquely identifies a cell, of a cell or target cell. In order to acquire the CGI of the target cell, the UE has to read at least part of the system information, SI, including master information block, MIB, and the relevant system information block, SIB. The reading of SI for the acquisition of CGI is carried out during measurement gaps which are autonomously created by the UE. These UE created gaps are also referred to as autonomous gaps.
The UE created autonomous gaps may adversely affect several different measurements that the UE is required to perform.
For example, in case the autonomous gaps overlap with positioning measurements, the positioning performance may be degraded or, in worst case, the positioning will fail. This may happen since the periodicity of positioning occasions is relatively long (160, 320, 640, 1280 ms), which results in that positioning measurements occasions are sparse in time, which impacts the RSTD measurement reporting time, but also the RSTD measurement accuracy.
In another example, the E-CID measurements, e.g. UE Rx-Tx, eNodeB Rx-Tx or RSRP/RSRQ, performed at specific measurement patterns, e.g. in subframes indicated for measurement by eICIC restricted measurement pattern, will degrade if the measurement occasions will collide with UE-configured autonomous gaps, in particular for patterns with low blanking rate i.e. when the number of measurement occasions indicated by the pattern is relatively small.
In yet another example, the accuracy of the UE timing advance, which is a function of the UE and eNodeB Rx-Tx time difference measurements, may be degraded due to improper configuration of autonomous gaps.
In still another example, the minimum RLM and RRM requirements with eICIC are specified for restricted measurement patterns with low blanking rate ( 1/10, i.e. 1 out of 10 subframes in a frame is a low-interference subframe configured for the measurement). Autonomous gaps colliding with the sparse measurement occasions indicated by the restricted measurement pattern will degrade the RLM/RRM measurement performance due to a further reduced number of measurement possibilities.
In yet an example, the autonomous gaps are also created in the uplink when the UE reads the SI. Hence performance of the measurements (e.g. UE or eNodeB time difference measurements) which involve measurement on signals transmitted in the uplink when UE is reading SI can be deteriorated. In case the UL restricted subframes (or time-frequency resources) are used for uplink measurements, the impact of autonomous gaps on the measurement may be even severe. For example the collision or overlapping of the autonomous gaps with the UL restricted sub-frames (or time-frequency resources) may result in bad measurement performance.